January 1, lb95.] 



KNOWLEDGE. 



11 



wave-length is allowed to pass the second slit ; the light, 

 therefore, is far feebler than that used in the ordinary 

 photo-heliograph, where the entire light of the sun, undis- 

 persed, is used. What is required, then, is to slowly move 

 the entire spectroscope, the first slit travelhng across the 

 sun's image, the second across the plate, and an image of 

 the sun as given by light of the wave-length in question is 

 built up on the plate. In practice there are several ways 

 of effecting this result, one of the most convenient being 

 to fix the spectroscope, allowing the sun to transit across 

 the first slit while the plate itself is moved by clockwork at 

 the recjuisite speed behind the second slit. 



In this way Mr. Hale and ^I. Deslandres have supplied 

 us with photographs of the sun produced by the light of 

 the K line, the new method having suggested itself to them 

 from the fact that their photographs of the solar spectrum 

 showed that the H and K lines were bright in the spectra 

 of faculie. And, as is shown by the two upper photographs 

 of the four in the accompanying plate, such photographs 

 display a network of bright markings extending across the 

 disc, and closely resembling in character the facuLe which 

 we see directly in the telescope, or photograph in an 

 ordinary photo-heliograph. 



Moreover, a comparison of these two photographs with 

 those taken on tlie same days, 18!)I, April 10th and 11th, 

 at (Ireenwich, shows that to a certain extent the ordinary 

 facula? and these bright K line districts actually correspond. 

 Thus the bright region at about position- angle 24:0^(from the 

 north extremity of the sun's axis, »(<r from the north point) 

 on Fig. 1, and still closer to the limb on Fig. 2, corre- 

 sponds in position to a group of facals on the Greenwich 

 photographs. So, again, the bright region a little to the 

 north-east of the one just mentioned has its correlative 

 on the plates taken with the ordinary photo-heliograph. 

 Representatives can also be traced of the great stream of 

 brilliant K matter in N. lat. 17° or 1H°, and stretching 

 from the central meridian almost to the west limb. And 

 the bright cluster about position-angle 11.5° is also repre- 

 sented, and represented not merely as to general position, 

 but as to form and extent. 



But this correspondence between the bright regions on 

 the photographs taken with the photo-heliograph and the 

 spectro-heliograph is not exact. The spectro-heliograph 

 does not merely show us what we should see if we made 

 a rapid circuit of the sun, and noted the facula? revealed 

 near the limb as the visible hemisphere shifted its boundary 

 to our gaze. The K line faculn; — if, from their appearance, 

 we may so call the faculous-looking regions on these 

 photographs — are not the same as the faculse shown on an 

 ordinary photograph. The two are clearly connected ; they 

 are as clearly not identical. 



To take the most striking instance in the photographs 

 before us, the great solid mass of K facuhij in the northern 

 hemisphere and on the central meridian. This region, on 

 the ordinary sun-picture, is the site of a fine train of spots, 

 stretching over 13° of solar longitude, and with a breadth 

 of 5|° of solar latitude, and an area of 860 millionths of 

 the hemisphere on April 10th, which has increased to 

 980 millionths by April 11th. ( )r, translating these elements 

 into English miles, the greatest length of the group is very 

 nearly 100,000 miles, its greatest breadth more than 

 •10,000 miles, its area on April 10th 1000 million square 

 miles, and on April 11th 1160 million square miles. 



This train of spots is, indeed, represented on M. 

 Deslandres' photographs by a few small spots, but the 

 great body of the train is covered up from us by the mass 

 of K facula'. 



Similarly, the group of K faculse first mentioned, near 

 position-angle 240°, is associated with a group of spots. 



which it partly covers. The K facula has a distinctly 

 larger area than the corresponding facula on the ordinary 

 sun-picture, and as the facula is close to the limb we cannot 

 ascribe its smaller area on the Greenwich photograph to 

 its unfavourable position. If it had been of equal area on 

 the two photographs it would have appeared to us so. 



The K faculffi shown us by the spectro-heliograph are, 

 therefore, not merely the ordinary faculre, but are some- 

 thing more. Since ordinary faculs have the K line bright 

 in their spectra, the spectro-heliograph euables us to photo- 

 graph them, not merely near the limb, but wherever they 

 may be on the disc. But over and above such faculre, 

 it shows us another class of faculae more extensive than 

 the first. 



The difference between the two classes is this. The 

 ordinary facuhe shine principally with white light, they 

 give out chiefly a continuous spectrum — a light and a 

 spectrum in itself feebler than that of the photosphere, 

 but appearing to be brighter when seen near the limb, 

 because the facuL-e lie above the greater part of that 

 layer of relatively dark dust which causes the enfeeble- 

 ment of the light near the sun's limb. They thus escape 

 obscuration by it. For the most part they probably consist 

 of incandescent solid matter, no doubt in a finely divided 

 state ; and because solid, they are opaque. An ordinary 

 facula, crossing a spot, effectually hides ffom our view the 

 region beneath it. 



The K line facuhe, on the other hand, consist of glowing 

 gas. They cover all the region occupied by the first clasi, 

 but they also extend beyond, and especially so in the 

 region of spots; and unlike ordinary facnlfe, they do not 

 hide the spots lying beneath them. 



These spectrographs give us yet further iaformation. 

 For the K line is given under a triple form corresponding 

 to three different layers of the solar atmosphere over the 

 greater part of the disc. First, a very broad dark line; 

 then a bright line superposed on the first, and not so 

 broad ; thirdly, a narrow dark line superposed on the 

 second line, and so dividing it into two. The broad dark 

 line corresponds to the " reversing layer," ia close contact 

 with the photosphere ; the bright line to a region just 

 above, i.e., to the lowest hottest stratum of the chromo- 

 sphere, the region represented, therefore, in M. Deslandres' 

 photographs ; the narrow dark line in the centre, to a 

 region higher still. Over the spots, however, it is usual 

 to find the bright K line single, not double ; corre- 

 sponding, that is, to the uppermost of tlie three layers. 

 Not that it follows that the calcium vapour over a spot is 

 higher than over the general disc ; rather the reverse, 

 that we have here a region of lowered temperature and 

 pressure, and that the conditions necessary for the pro- 

 duction of the broader bright line of the second layer are 

 not present here. 



The spectro-hehograph not merely reveals to us these 

 K line faculae over the disc, it shows us the chromosphere 

 and prominences beyond the limb, and the originals_ of 

 Figs 1 and 2 show a number of prominences surrounding 

 the sun ; but the exposure having been calculated for the 

 general disc, they are only faint, and have been lost in the 

 reproduction. Figs. 8 and 4, however, are copies of 

 photographs taken on the same days as the first two, but 

 with fuller exposure, and they accordingly show the 

 prominence forms very distinctly. The central portion of 

 the sun was shutout by a diaphragm, very slightly smaller 

 than the solar image." The most interesting prominence 

 is the one near the south pole, which has changed in so 

 remarkable a manner between the times that the originals 

 of Figs. 3 and 4 were taken ; indeed, a great change took 

 place between the time of Fig. 2, taken April lltli, lib. 



